The 5-HT(3) subtype of a serotonin receptor is unique in that it is directly coupled to a ligand-gated ion channel and functions in rapid synaptic transmission in the brain. The functional properties of this receptor are important for understanding the role serotonin plays in brain function. One functional aspect of the 5-HT(3) receptor investigated is the phenomenon of desensitization, whereby the 5-HT(3) receptor channel opens and then closes in the continued presence of 5-HT. Desensitization is thought to be an important characteristic of ion channels that function in rapid transmission as a way for communication between cells to be short in duration with the potential for higher frequency communication. Many factors regulate the rate of desensitization, including the primary amino acid structure of the receptor channel, membrane voltage, calcium, and the phosphorylation state of the channel. I am studying how these and other factors control the rate of desensitation of the 5-HT(3) receptors expressed in Xenopus oocytes, using voltage-clamp techniques. I have recently reported work at NIEHS that greatly expanded upon my earlier work to show how complex the kinetics parameters of desensitization are effected when a particular leucine residue found at position 286 of the native 5-HT(3) channel is mutated to threonine, and how the rate of desensitization is altered by external calcium ions. Intracellular calcium also profoundly effects the function of the 5-HT(3) channel since chelating internal calcium greatly reduces the rate of desensitization. Recent results suggest that protein phosphatases may be playing a very significant modulatory role. In another series of experiments, we demonstrated that the 5-HT(3) channel is not permeable to calcium, a hotly-debated point with major significance. As calcium entry and regulation in cells has profound effects on many profound effects on many signal transduction pathways, the isse of calcium permeability of ion channels is very important. The aim of this work is to determine what parameters affect when ligand-gated ion channels open and close, and how this is modulated.